Mesh triangle, quad, and n-gon
1) 网格三角面有三个边。只能是平面的三角面是最基本的网格面。
2) 网格四边面有四个边。它由两个三角面组成，且内部边缘不可见。
3) A mesh n-gon has more than four edges. It consists of more triangles with all interior edges invisible.